Gas turbine engine components may be formed of superalloy material known for high temperature performance in terms of tensile strength, creep resistance, oxidation resistance, and corrosion resistance. The superalloy component may be a nickel-base alloy, wherein nickel is the single greatest element in the superalloy by weight. Illustrative nickel-base superalloys include at least about 40 wt % Ni, and at least one component from the group including cobalt, chromium, aluminum, tungsten, molybdenum, titanium, and iron.
Various turbine engine components crack, erode or experience conditions necessitating a repair. No joining process (braze, bond, weld, etc) will have a good result with contaminants/oxides present; e.g. brazes won't adhere, welds will have defects etc. In situations where the contaminant or oxide cannot be removed by a simple mechanical means due to its location in a crevice or crack, special cleaning techniques are required. Note that the term contaminant may be used herein to include both oxides and non-oxides, although oxide contaminants are commonly found in gas turbine applications.
Prior to servicing a turbine component, it is necessary that contaminants/oxides be removed so that a subsequent braze, for example, will adhere to the base material. A fluoride ion cleaning (FIC) procedure currently known in the industry utilizes hydrofluoric acid (HF) at elevated temperatures converting metal oxides to gaseous metal fluorides and water. Because hydrofluoric acid is an extremely corrosive acid, it is known that the acid may impede servicing by degrading an existing base material by depleting compositional elements and/or causing intergranular attack. Furthermore, hydrofluoric acid is extremely dangerous to handle and may cause skin injury or corneal damage. U.S. Pat. No. 7,303,112 describes a method of repairing a braze joint which includes the use of both an alkali metal molten salt bath and an acid solution.
Thus, there is an ongoing need for an improved turbine component repair procedure incorporating a safe and effective cleaning process.